This invention relates to oxazolidone ring-containing modified epoxy resins.
The invention also relates to cathodic electrodeposition paints containing said modified epoxy resins.
Modified epoxy resins to be formulated in cathodic electrodeposition paints are conventionally produced by reacting an epoxy resin with a first active hydrogen compound such as alkyl phenols, monocarboxylic acids or hydroxycarboxylic acids to open a portion of the epoxide rings, and then with a second active hydrogen compound such as primary or secondary amines to open the remaining epoxide rings. The resulting modified resins are combined with a crosslinker such as melamine resins or blocked organic polyisocyanates, applied electrically onto a substrate, and baked at an elevated temperature to give a cured film. Modified epoxy resins of this type have certain disadvantages in that they react with the crosslinker only at relatively high temperatures. It is also known to make the modified epoxy resins self-crosslinkable by attaching a half blocked diisocyanate to a secondary alcoholic hydroxyl group formed by the ring-opening reaction of the epoxy ring. This resin also suffers from certain disadvantages in that the blocked isocyanate moiety bound through the urethane linkage tends to be liberated at least partially when the film is baked at a temperature above 200.degree. C.
Commonly assigned U.S. Pat. No. 5,276,072 discloses a hydrophilic resin having an oxazolidone ring-containing backbone and an ionizable group at the terminals of the backbone. The resin backbone is produced by reacting an organic diisocyanate and an epoxy resin at an epoxy/isocyanate equivalent ratio greater than 1 so that all of the isocyanato groups are converted to the oxazolidone rings.
We have now found that by reversing the epoxy/isocyanate equivalent ratio, it is possible to produce an epoxy resin-diisocyanate adduct in which only one isocyanato group has been converted to the oxazolidone ring and the other isocyanato group has been reversibly blocked or otherwise modified in a useful manner.